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"Although our numbers are still being calculated, we were able to screen over 75 children at the Pediatric Foundation of Guatemala and we operated on 44 children, many of (who or which) received multiple procedures."
Either "who" ( colloquial,not recommended in written form)or "whom" (recommended by grammarians) since it´s a relative pronoun that refers to the children who(m) have undergone multiple procedures. (Poor kids! )